1. Field of the Invention
The present invention relates to an ultrafast integrated circuit used for ultra-wide-band communications for example and a technique of generating an ultrashort pulse required for the communications, and more particularly, to a pulse polarity modulation circuit for converting a unipolar pulse into a bipolar pulse.
2. Description of the Related Art
There is a general requirement in many communication devices, to convert a unipolar signal into a bipolar signal. It is desirable for this kind of unipolar/bipolar converter to be as small as possible and have low power consumption and a low-noise characteristic.
Particularly, in ultra-wide-band communications as referred to in the present invention, an ultrashort pulse whose full width at half maximum is not more than 10 ps is generated and a unipolar pulse is converted into a bipolar pulse as required. It is desirable for this kind of pulse circuit to be a differential circuit which has exceptional switching speed.
The following document describes a conventional example of the unipolar/bipolar converter.
Patent document 1: Published Japanese Patent Application No. 3-27622 “U/B CONVERTER”
This document discloses a circuit for converting a unipolar signal into a bipolar signal with the outputs of dual-circuit pulse generators combined. FIG. 1 is a block diagram of a unipolar/bipolar converter similar to this circuit. In FIG. 1, the outputs of pulse generators which generate a positive-polarity signal and a negative-polarity signal respectively are combined and output through an output buffer.
FIG. 2 is an example of an operation time chart of the circuit in FIG. 1. Positive-polarity pulses a and negative-polarity pulses b are combined, and bipolar pulses Q are obtained. Here, positive-polarity pulses a and negative-polarity pulses b are generally different in frequency.
In the converter of patent document 1, however, there is a problem that it is difficult to realize low power consumption and space saving because dual-circuit pulse generators are required. Further, as a pulse circuit for ultra-wide-band communication, it is desirable to use the differential circuit as described; however, the gain of the differential circuit attains a maximum around a logic threshold value which is the logic mid-point (0 point); therefore, if minute noise occurs around the middle level, there is a problem that the noise is amplified to a large degree by an amplifier circuit generally placed at a subsequent stage, and additional circuit design is required to keep the middle level flat.